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Old 29th November 2005, 12:04 PM   #1
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Default Idea to attract more flux thru an underhung coil

I was just wondering if it would be a good idea to attach a very thin sheet of iron around a coil within an underhung top plate. This would direct more flux through the coil and improve efficiency. Possible problems that I can see are:

- tiny increase in moving mass,
- coil may be prone to being attracted to the pole pieces. The air gap would have to be slightly bigger to compensate. (also because coil is slightly thicker).

Maybe the reduction in flux by this increase in gap would make it not worth it.

Someone has no doubt already thought of this. Have any of you ever come across it before? Any thoughts as to whether you think it is worth it?

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Old 29th November 2005, 03:04 PM   #2
Ron E is offline Ron E  United States
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Won't work cause iron is conductive and it will act as a shorting coil. if you put a slot in it up one side it will still act as a potent eddy current brake and reduce Qms. I'd guess you could model how effective it is at "focusing" flux with FEMM. I'd be concerned about added mass, etc.

I fail to see the attraction in underhung designs, their flux vs excursion curve isn't inherently more linear than an overhung design. One benefit I see is less inductance modulation, but that is a second or third order (in importance) effect and Cms/Bl nonlinearity is more important for low frequencies. For high frequencies the excursion isn't that great so it is less of a factor..... It would be interesting to know the relative values of the distortion caused by inductance modulation in overhung and distortion caused by the gap not being saturated in underhung designs.

Even TC Sounds, the company that made their name making underhung designs, mostly uses overhung for high excursion woofers now.
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Old 29th November 2005, 03:14 PM   #3
Vikash is offline Vikash  United Kingdom
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Simply make the air gap smaller. Adding a heavy conductive metal to the moving mass only has drawbacks as far as I can see.
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Old 29th November 2005, 04:33 PM   #4
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Quote:
Originally posted by Ron E
Won't work cause iron is conductive and it will act as a shorting coil. if you put a slot in it up one side it will still act as a potent eddy current brake and reduce Qms. I'd guess you could model how effective it is at "focusing" flux with FEMM. I'd be concerned about added mass, etc.

I fail to see the attraction in underhung designs, their flux vs excursion curve isn't inherently more linear than an overhung design. One benefit I see is less inductance modulation, but that is a second or third order (in importance) effect and Cms/Bl nonlinearity is more important for low frequencies. For high frequencies the excursion isn't that great so it is less of a factor..... It would be interesting to know the relative values of the distortion caused by inductance modulation in overhung and distortion caused by the gap not being saturated in underhung designs.

Even TC Sounds, the company that made their name making underhung designs, mostly uses overhung for high excursion woofers now.



as far as subs are concerned you're correct, BL/Kms is the biggest issue... and 70% of the problems are BL related

modulation with inductance is something more problematic to midranges and domes... other than that it's really a non issue for lower frequency drivers where distortions need to approach 3-4% before they become audible... and generally on the higher orders

underhung isn't necessary "better" and overhung isn't necessarily "better".... I don't see any advantages for either design unless you are looking for speakers above 500hz... and I would just go underhung as you don't need a lot of excursion anywho
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